1. Field of the Invention
The present invention relates to a apparatus made of silica for semiconductor device fabrication, and particularly to a apparatus made of silica such as a furnace tube, wafer boat or stick used for fabricating semiconductor devices.
2. Description of the Related Art
With the progress of the high integration for semiconductor integrated circuit devices, there has been required such a fabrication process as to exclude the contamination causing the deterioration in the device characteristics. An clean atmosphere without any metal contamination caused by alkali metals such as Na and K, and transition metals such as Fe and Cr is required for a fabrication process for semiconductor devices.
In particular, in a heat treatment process over about 900.degree. C., if a semiconductor wafer is exposed to an atmosphere containing contaminated metals, the metals tend to be introduced in the semiconductor wafer, resulting in the deterioration of the semiconductor device fabricated using the semiconductor wafer.
Accordingly, the materials for jigs used in the heat treatment process particularly require high purity, that is, the reduction in the amount of metal impurities contained therein.
In a heat treatment process for a semiconductor wafer used as a substrate of a semiconductor device and in a film deposition process for forming films on the surface thereof, jigs made of silica, SiC and the like are used. These materials are also required to be reduced in the contents of the impurities.
As a furnace tube used in a film deposition apparatus and a heater, there is used such a cylindrical silica tube c as shown in FIGS. 1(A) and 1(B). The silica tube c contains a wafer boat made of silica b mounting a semiconductor wafers w.
For fabricating a silica constituting the silica tube c or the wafer boat b, there have known two methods: (1) an electrical fusion method; and (2) a oxyhydrogen flame fusion method. These methods are similar to each other in using natural silica rock or silica sand as the raw material.
The silica formed by the oxyhydrogen flame fusion method is worse in the heat resistance and tends to be deformed by the applying of heat. The electrical fusion method has been developed for solving the disadvantage of the oxyhydrogen flame fusion method. The silica formed by the electrical fusion method is,excellent in the heat resistance.
The reason for this is that the amount of the hydroxyl group (OH group) contained in the silica formed by the electrical fusion method is smaller than that contained in the silica formed by the oxyhydrogen flame fusion method. The detail has been reported, for example, in "NEW GLASS" No. 14. 1987 (New Glass Forum), "Industrial Material": The Daily Industrial News, September 1974.
The above two methods produce the silica using the natural silica rock or silica sand as the raw material. The silica thus obtained contains contaminated elements in a large amount. Accordingly, it is required to suppress the release of these contaminated elements to the outside of the silica.
A technique for suppressing the release of the contaminated elements has been proposed in Japanese Patent Laid-open Nos. sho 55-126542 and hei 3-84922. In this technique, a silica tube is so constructed that a high purity synthesis silica layer by vapor deposition is formed on the inner peripheral surface of a hard silica tube formed by the electrical fusion method. This is intended to prevent the internal diffusion of impurities by the synthesis silica layer.
However, the synthesis silica layer is small in the content of impurities but is large in the content of the OH group. As a consequence, the synthesis silica layer is small in its viscosity and is worse in the heat resistance, thus causing such an inconvenience that it tends to be fused to the other members when exposed to high temperatures.
Also, in Japanese Patent Laid-open No. sho 63-236723, there has been proposed a structure that a surface layer containing an aluminum element is formed on the outer peripheral surface of the silica tube formed by the electrical fusion method. This is intended to obstruct the permeation of the impurities from the outside by the surface layer.